Sediment cores from salmon nursery lakes on Alaskas Kodiak Island show
that spawning records crashed for hundreds of years prior to modern human intervention.
Canadian and American researchers suspect climate is the cause.

In recent
years scientists have recognized that, along with commercial fishing and other
human activities, climate is a significant factor in the survival of adult salmon.
Indeed, climate is one of many variables fish managers are trying to incorporate
into their models for predicting how many salmon will make it back from the
ocean to their natal rivers and lakes to spawn. The most reliable method to
date, is to count how many salmon return after spending only a year in the ocean.
If conditions are good, the siblings of those first-year salmon will spend a
longer time in the ocean and also have a strong survival rate.

Climate and ocean conditions together form the backdrop on which the salmon
can thrive or not thrive depending on the stage salmon are in, and all other
things fall on that potential, says Oregon State climatologist George
Taylor.

Bruce Finney (pictured) and colleagues
use lake sediment cores to identify the productivity of sockeye salmon during
the last 2,000 years. Photo by Jim Larson, U.S. Fish & Wildlife Service

Until now, swings in salmon population linked to climate were known to last
only decades, not centuries. But the longest track record for salmon went back
only 300 years. Bruce Finney of the University of Alaska in Fairbanks and colleagues
had set that record two years ago using lake sediment cores. Finney had followed
the unique life cycle of sockeye salmon. Unlike other salmon species, sockeye
return to lakes rather than rivers to spawn and die. There, the sediment collects
over time without washing away. By taking deeper lake sediment cores, the team
have extended their knowledge of salmon populations back to 2,200 years ago.
They reported their findings in the April 18 Nature. About a meter in
length, the cores gave the team a new perspective on the highs and lows of sockeye
salmon productivity in the northeastern Pacific Ocean.

Locked in the sediments of Alaskas Karluk and Akalura lakes are chemical
and biological indicators that reflect past abundances of sockeye salmon spawners.
Sockeye will sometimes spend up to four years in the Pacific Ocean feeding on
krill, squid and other fish. With their diet comes a body mass loaded with nitrogen-15,
a stable nitrogen isotope. The fluctuations in the number of salmon returning
to the nursery lakes left signals the scientists read like tree rings in the
freshwater environment. After spawning a new brood, the adult salmon would die
and litter the lake bottom with nitrogen-15 and other nutrients that would provide
sustenance for zooplankton and eutrophic diatoms, a group of algae characteristic
of a high- nutrient environment. These in turn provided baby food for the newly
hatched salmon waiting to continue the cycle with their own trip to the ocean.
By contrast, a neighboring lake without salmon showed consistently low levels
of nitrogen-15 and oligotrophic diatom species, typically found in low-nutrient
lakes.

Finney and his colleagues analyzed every half centimeter of the cores for nitrogen-15
and diatom species, scooping from the mud the equivalent of what would fall
during a 20-year period of deposition. It was a great surprise to see
the dynamics have changed quite a lot, says Marianne Douglas of the University
of Toronto in Ontario, Canada. She was shocked to see salmon abundance fall
to such dramatic lows without human influence. Its the first time
weve seen good hard data that huge dynamics in the population occurred
without being altered by human activities.

They found that the population of sockeye salmon in Karluk lake around 200 B.C.
was about as high as it was when commercial fishing began at the Karluk estuary
in 1882: about 3 million. But beginning around 100 B.C. the return of salmon
to the lake steadily decreased, hitting a record low of about 100,000 or 200,000
fish around A.D. 250. The population slowly crept back up, but remained well
below the average mean for another 550 years. Not until A.D. 1200 did the salmon
reach the 3 million mark again. From then until the start of commercial fishing,
the return of salmon to Karluk lake remained relatively high. Over the last
100 years the numbers have again decreased dramatically; the authors attribute
this dip to both commercial fishing and climatic changes.

But the climatic influence that may have caused the centuries-long, low productivity
2,000 years ago is still not clear. The authors note that around 100 B.C. a
warming of marine waters occurred in the Santa Barbara basin in California,
presumably lowering ocean productivity in the north. The rebound of salmon productivity
in A.D. 1200 corresponded to a period of glacial advances in southern Alaska
and the Canadian Rockies. More work is needed to try and delineate what
is the relationship between climate and salmon beyond just colder waters lower
productivity in the Gulf of Alaska. If the ocean is too warm thats not
too good either, says Irene Gregory-Eaves of Queens University in
Kingston, Ontario, a co-author of the Nature report. If we can nail that
down in a more definitive way and get together with those working on climate
projections then we can better understand the response of salmon to climate
changes in the future.

The concern is if a climatic shift topples the natural productivity levels at
a time when human impacts have already brought the salmon population to another
low. Adds Finney: The global warming projections indicate that they may
see changes that could be more extreme and sudden than in the past.
Christina Reed